1. Field of the Invention
The invention relates to a process for the preparation of magnetite particles and to their use.
2. Brief Description of the Prior Art
Particulate magnetites which are prepared from aqueous solutions by a precipitation process have long been known. U.S. Pat. No. 802 928 describes the preparation of magnetite by precipitation of iron(II) sulphate using an alkaline component and subsequent oxidation with air. In numerous further, subsequent publications, the preparation of magnetites by the precipitation process is likewise described. The preparation of magnetites by the precipitation process with addition of silicon is described in JP-A 51 044 298. Pure precipitated magnetites without addition of foreign elements can be prepared batchwise according to DE-A 3 209 469 or continuously according to DE-A 2 618 058. Usually, FeSO4 is used as the iron(II) salt. However, it is also possible to use any soluble iron(II) salt for the preparation of a magnetite by the precipitation process. The use of FeCl2, as described in DE-A 3 004 718, is particularly suitable here. The use of FeSO4 or FeCl2 has the advantage that both substances &an be obtained in large amounts very economically as waste substances from the iron-processing industry. A suitable precipitating agent in addition to the most frequently used sodium hydroxide is also CaO or CaCO3 (DE-A 3 004 718), ammonia (DE-A 2 460 493) or Na2CO3, MgCO3 or MgO (EP-A 0 187 331). As a rule, air is used as an oxidizing agent. However, processes for oxidation with nitrates (DD-A 216 040 and DD-A 284 478) are also described.
The magnetites were initially used for the preparation of paints of all types. The particular advantage of magnetites over organic dyes and carbon black lies in their very much better weather resistance, so that paints containing magnetite can also be used outdoors.
Furthermore, precipitated magnetites are popularly used for colouring shaped concrete articles, such as, for example, concrete slab stones or concrete roof tiles.
Magnetites have for some time also been used for the preparation of toners in electrophotography. Magnetites which were prepared by the precipitation process are preferably used for the preparation of toners for copiers using one-component toners. The magnetic toners used for this purpose must have various properties. With progressive development and improvement of the copiers and printers, the requirements which the magnetic toners and consequently the magnetite used for this purpose have to meet have become increasingly high. The latest printer generation achieves a resolution of more than 400 dpi (dots per inch), for which the development of finely divided toners having a very narrow particle size distribution was required. As a result of this, the magnetites used for this purpose likewise have to have a very narrow particle size distribution. Furthermore, a specific particle size is required so that homogeneous distribution of the magnetite particles in the prepared toner is ensured. The magnetites themselves. Must have sufficiently high electrical resistance to stabilize the latent image during the electrostatic transfer. Furthermore, coercive force, saturation magnetization and especially the remanent magnetization must be in the correct ratio to the field strengths prevailing in the machine.
For use in magnetic toners, Si-containing magnetites are particularly frequently employed. These have a charge behaviour differing from that of pure magnetites and, with the same particle size, have higher thermal stability. A process for the preparation of such particles is described in JP-A 61 034 070. Here, the Si-component is added to the iron(II) sulphate, which however leads to precipitates of silica and hence to a non uniform distribution of the silicon in the magnetite lattice. U.S. Pat. No. 4,992,191 describes a magnetite containing 0.1 to 5.0 atom % of Si, based on Fe, which is said to be particularly suitable for the preparation of toners.
In the process described there, a silicate component is added to an alkaline component in the form of an aqueous solution, and then an iron(II) component in the form of an aqueous solution is added in an amount such that a molar ratio of Fe(II) component to alkaline component of about   0.53  ⁡      [          1.5      2.85        ]  is present, the temperature being kept at 90° C. The suspension thus obtained is then treated with air as an oxidizing agent in order to obtain spheroidal, silicon-containing magnetite having a particle size in the range of 0.1 to 1.0 μm. The particles obtained are filtered, washed and milled.
DE-A 19 702 431 describes a further process for the preparation of particularly round Si-containing magnetites. There, the preparation of Si-free round magnetites is also described for the first time in detail in a comparative example. However, the thermal stability of these magnetites is described as being insufficient for the preparation of magnetic toners.
It was the object of the present invention to develop a process for the preparation of round low-Si magnetites (Si content less than 0.025% by weight), which are particularly suitable for newly developed copiers and laser printers. In the current generation of copiers and laser printers, magnetites having relatively high coercive forces (3 979 to 7 162 kA/=40 to 90 Oe) are increasingly required. By using low-Si magnetites, the charge behaviour can be adjusted using charge additives independently of the magnetite. The flow properties of the toner prepared using such magnetites can likewise be adjusted using flow improvers (typically finely divided silica), independently of the inherent flow behaviour of the magnetite.
It was surprisingly found that the magnetites described in DE-A 19 702 431 as being insufficiently thermally stable can be very readily used in toners. Moreover, the preparation process is more economical owing to the omission of the component introducing the Si, with the result that a process step is also dispensed with.
This object could be achieved by the magnetites according to the process according to the invention. These magnetites can not only be used in magnetic toners but also for colouring paper, plastic, finishes, fibres and concrete and in paints.